Avocado (Persea americana).
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Avocados are a commercially valuable crop whose trees and fruit are cultivated in tropical climates throughout the world (and some temperate ones, such as Southern California), producing a green-skinned, pear-shaped fruit that ripens after harvesting. Trees are partially self-pollinating and often are propagated through grafting to maintain a predictable quality and quantity of the fruit.
P. americana has a long history of being cultivated in Central and South America; a water jar shaped like an avocado, dating to A.D. 900, was discovered in the pre-Incan city of Chan Chan though there is evidence of cultivation in Mexico for as long as 10,000 years. The earliest known written account of the avocado in Europe is that of Martín Fernández de Enciso (c. 1470–c. 1528) in 1518 or 1519 in his book, Suma de Geografía que Trata de Todas las Partidas y Provincias del Mundo. The first written record in English of the use of the word 'avocado' was by Hans Sloane in a 1696 index of Jamaican plants. The plant was introduced to Indonesia by 1750, Brazil in 1809, the Levant in 1908, and South Africa and Australia in the late 19th century.
|Food and agriculture|
|Country||Quantity (Tm)||World Rank1|
|United States of America||214,000||3|
|1Source: FAO (2004) Major Producers of Avocado|
The subtropical species needs a climate without frost and with little wind. High winds reduce the humidity, dehydrate the flowers, and affect pollination. In particular, the West Indian type requires humidity and a tropical climate which is important in flowering. When even a mild frost occurs, some fruit may drop from the tree, reducing the yield, although the Hass cultivar can tolerate temperatures down to −1°C. The trees also need well aerated soils, ideally more than 1 m deep. Yield is reduced when the irrigation water is highly saline. These soil and climate conditions are provided only in a few areas of the world, particularly in southern Spain, the Levant, South Africa, Peru, parts of central and northern Chile, Vietnam, Indonesia, Sri Lanka, Australia, New Zealand, the United States, the Philippines, Malaysia, Central America, the caribbean and Mexico, the center of origin and diversity of this species. Each region has different types of cultivars. Mexico is the largest producer of the Hass variety, with over 1 million tonnes produced annually.
The avocado is a climacteric fruit (the banana is another), which means that it matures on the tree but ripens off the tree. Avocados used in commerce are picked hard and green and kept in coolers at 38 to 42°F (3.3 to 5.6°C) until they reach their final destination. Avocado must be mature to ripen properly. Avocados that fall off the tree ripen on the ground, and depending on the amount of oil they contain, their taste and texture may vary greatly. Generally, the fruit is picked once it reaches maturity; Mexican growers pick Hass-variety avocados when they have more than 23% dry matter and other producing countries have similar standards. Once picked, avocados ripen in a few days at room temperature (faster if stored with other fruits such as bananas, because of the influence of ethylene gas). Premium supermarkets sell pre-ripened avocados treated with synthetic ethylene to hasten the ripening process. In some cases, avocados can be left on the tree for several months, which is an advantage to commercial growers who seek the greatest return for their crop; if the fruit remains unpicked for too long, however, it will fall to the ground.
The species is only partially able to self-pollinate, because of dichogamy in its flowering. This limitation, added to the long juvenile period, makes the species difficult to breed. Most cultivars are propagated via grafting, having originated from random seedling plants or minor mutations derived from cultivars. Modern breeding programs tend to use isolation plots where the chances of cross-pollination are reduced. That is the case for programs at the University of California, Riverside, as well as the Volcani Centre and the Instituto de Investigaciones Agropecuarias in Chile.
The avocado is unusual in that the timing of the male and female flower phases differs among cultivars. There are two flowering types, "A" and "B". "A" cultivar flowers open as female on the morning of the first day and close in late morning or early afternoon. Then they open as male in the afternoon of the second day. "B" varieties open as female on the afternoon of the first day, close in late afternoon and reopen as male the following morning.
Certain cultivars, such as the Hass, have a tendency to bear well only in alternate years. After a season with a low yield, due to factors such as cold (which the avocado does not tolerate well), the trees tend to produce abundantly the next season. This heavy crop depletes stored carbohydrates, resulting in a reduced yield the following season, and thus the alternate bearing pattern becomes established.
Unlike citrus fruits, rodents are attracted to the avocado tree and fruit during breeding.
While an avocado propagated by seed can bear fruit, it takes roughly 4–6 years to do so, and the offspring is unlikely to resemble the parent cultivar in fruit quality. Thus, commercial orchards are planted using grafted trees and rootstocks. Rootstocks are propagated by seed (seedling rootstocks) and also layering (clonal rootstocks). After about a year of growing the young plants in a greenhouse, they are ready to be grafted. Terminal and lateral grafting is normally used. The scion cultivar will then grow for another 6–12 months before the tree is ready to be sold. Clonal rootstocks have been selected for specific soil and disease conditions, such as poor soil aeration or resistance to the soil-borne disease caused by phytophthora (root rot).
While dozens of cultivars are grown in California, the Hass avocado is today the most common. It produces fruit year-round and accounts for the majority of cultivated avocados in the US. All Hass avocado trees are descended from a single "mother tree" that was raised by a mail carrier named Rudolph Hass, of La Habra Heights, California. Hass patented the productive tree in 1935. The "mother tree", of uncertain subspecies, died of root rot and was cut down in September, 2002.
After the North American Free Trade Agreement (NAFTA) went into effect in 1994, Mexico tried exporting avocados to the US. The US government resisted, claiming that the trade would introduce fruit flies that would destroy California's crops. The Mexican government responded by inviting US Department of Agriculture inspectors to Mexico, but the U.S. government declined, claiming fruit fly inspection is not feasible. The Mexican government then proposed to sell avocados only to the northeastern US in the winter (fruit flies cannot withstand extreme cold). The US government balked, but gave in when the Mexican government started throwing up barriers to US corn.
Legitimate pest invasion issues exist, as avocado pests originating in Mexico have made their way to California, including the persea mite and avocado thrips. These pests have increased pest control costs and made previously-relied-upon biological control less feasible. Other potentially disastrous pests, including a weevil, remain risks. Another argument is that the lower prices generated by Mexican (and Chilean) imports would increase the popularity of avocados outside of California, thereby assuaging the loss of profits due to the new competition.
Today avocados from Mexico are allowed in all 50 states. This is because USDA inspectors in Michoacán (the Mexican state where 90% of Hass avocados from Mexico are grown), have cut open and inspected millions of fruit in Uruapan, finding no problems. Imports from Mexico in the 2005–2006 season exceeded 130,000 tonnes.
Avocados are more expensive in the US than in other countries, because those consumed in the US are grown almost exclusively in California and Florida. California produces about 90% of the nation's avocado crop.
The avocado is very popular in vegetarian cuisine, making an excellent substitute for meats in sandwiches and salads because of its high fat content. The fruit is not sweet, but fatty, distinctly yet subtly flavored, and of smooth, almost creamy texture. It is used as the base for the Mexican dip known as guacamole, as well as a filling for several kinds of sushi, including California rolls. Avocado is popular in chicken dishes and as a spread on toast, served with salt and pepper. In Brazil and Vietnam, avocados are frequently used for milk-shakes and occasionally added to ice cream and other desserts. In Brazil, Vietnam, the Philippines and Indonesia, a dessert drink is made with sugar, milk or water, and pureed avocado. Chocolate syrup is sometimes added. In Australia it is commonly served in sandwiches, often with chicken.
In Mexico and Central America, avocados are served mixed with white rice, in soups, salads, or on the side of chicken and meat. In Chile its consumption is widespread and used as a puree in chicken, hamburgers and hot dogs, and in slices for celery or lettuce salads. The Chilean version of caesar salad contains large slices of mature avocado.
Avocado can be grown as a houseplant from seed. It can germinate in normal soil in a large pot or by suspending a washed pit in a glass (generally using toothpicks embedded in the sides) pointed-side up and filling the glass with water until the bottom quarter of the pit is covered. The pit will crack as it absorbs water and germinates, and should sprout in 4–6 weeks. When the roots and stem emerge from the seed, it can be planted in soil. Alternatively, vermicompost bins provide ideal conditions for germination of avocado pits. Once the pit sprouts a root transplant it to a pot containing a mixture of worm castings and potting soil. The stem should sprout within a month. The young tree is amenable to pruning and training but will not normally bear fruit indoors without sufficient sunlight and a second plant to cross-pollinate.